"We want to know on what basis they are still holding over 11,000 people in custody."
The army announced that 713 out of 12,000 Tamil Tiger combatants in detention were released at a ceremony attended by the president in the northern district of Vavuniya on Saturday.
The Tigers, who had fought for a separate Tamil homeland since 1972, were defeated by government troops eight months ago when the military killed guerrilla leader Velupillai Prabhakaran.
Between 80,000 and 100,000 people were killed in nearly four decades of ethnic bloodshed.
The United Nations estimated that 7,000 mainly Tamil civilians perished in the first four months of last year before the fighting eventually ended.
Rajapakse called the presidential election two years ahead of schedule in a bid to benefit from public acclaim for his government's victory over the Tigers.
Mahinda Rajapakse vowed to step up